A total of 37 strains ofPediococcuswere screened for antagonistic effect against the growth of eight strains ofListeria monocvtoaenes(CA, V7, Scott A, 19111, 19113, 19115, F5027, F5069). Using an agar plate diffusion method, 28% of the 296 pairings were positive with distinct, clear zones of inhibition evident. The positive reactions were primarily seen withPediococcus acidilacticistrains isolated from or associated with meat fermentations and from abdominal related sites in humans. Strains ofPediococcus pentosaceusandPediococcus damnosusalso showed an inhibitory effect againstListeria. Studies employing growth extracts ofP. acidilacticiPO2 versus growth ofL. monocvtoaenes19113 showed that the inhibitory substance was retained by a molecular sieve with a pore size of 12,000 daltons, that P02 growth extract in late stationary phase (48 hour culture) was most effective, and that apparent resistant sub‐populations of L.monocytoaenes19113 eventually arose after continued exposure to the growth extract.